The brake light switch may not be functioning properly. If it doesn't activate the brake lights, it also won't activate the shifter interlock. Check to see if you have brake lights. If not, have a look at the brake light switch and/or wiring.
Happened to me a few months ago. For me, the brake light fuse kept blowing whenever I stepped on the pedal (the lights are on the same circuit as the interlock switch for the gear shift). Figured out that: 1) there is a physical interlock just to the front left of the gear shift which you can trigger with a screwdriver to get the car out of park. It will drive just fine, etc... but you won't have brakelights, 2) the reason my fuse kept blowing was just really old brakelights that the terminals had begun to fuse on. Replaced the bulbs and everything is perfect again! Hope this helps.